Warning the rat off (6)
I believe the answer is:
threat
'warning' is the definition.
(I know that threat is a type of warning)
'the rat off' is the wordplay.
'off' indicates anagramming the letters.
'the'+'rat'='therat'
'therat' with letters rearranged gives 'THREAT'.
(Other definitions for threat that I've seen before include "An intimidating warning" , "Aggressive declaration" , "Menace of impending evil" , "A warning of harm to come" , "Endangerment" .)